This invention concerns thermoplastic polyurethane mouldings with very high surface resistance (resistance to writing, scratching and abrasion) and very good technical processability with a large processing window as well as their use.
Thermoplastic polyurethanes (TPU) are of great technical importance because of their good elastomer properties and thermoplastic processability. An overview of the production, properties and applications of TPU is given for example in the Plastics Handbook [G. Becker, D. Braun], volume 7 “Polyurethane”, Munich, Vienna, Carl Hanser Publishing, 1983.
TPU are made up mostly of linear polyols (macrodiols), such as polyester, polyether or polycarbonate diols, organic diisocyanates and short chain, mostly difunctional alcohols (chain lengtheners). They may be produced continuously or discontinuously. The best known production processes are the band process (GB-A 1 057 018) and the extrusion process (DE-A 19 64 834).
Thermoplastically processable polyurethane elastomers may be made up either step by step (prepolymer dosing process) or by the simultaneous reaction of all components in one stage (one shot dosing process).
In DE-A 102 30 020 the use of polyorganosiloxanes to improve resistance to scratching and rubbing (mechanical surface resistance) for TPU is described. Though when processing TPU, which contain these additives, after some time (after some shots) in the injection moulding process surface disturbances occur, which lead to undesirable, high rejection rates.
In EP-A 2 083 026 the use of special mixtures of low molecular and high molecular polyorganosiloxanes to improve resistance to scratching and rubbing (mechanical surface resistance) for TPU is described. Though when processing TPUs, which contain these additive mixtures, at low processing temperatures (<180° C.) in the injection moulding process surface disturbances and delamination occasionally occur, which lead to undesirable, high rejection rates.